1. Technical Field
The present invention relates to a resin composition and an optical compensation film using the same. More specifically, the present invention relates to a resin composition and an optical compensation film for a liquid crystal display, which is excellent in the retardation characteristics.
2. Background Art
A liquid crystal display is being widely used as a most important display device in the multimedia society, including applications from a cellular phone to a computer monitor, a laptop computer and a television set. In a liquid crystal display, many optical films are used so as to enhance display characteristics. Among others, an optical compensation film plays a great role in, for example, improving the contrast or compensating the color tone when the display is viewed from the front or oblique direction.
The liquid crystal display includes many systems such as vertical alignment type (VA-LCD), in-plane switching liquid crystal display (IPS-LCD), super twisted nematic liquid crystal display (STN-LCD), reflective liquid crystal display and transflective liquid-crystal display, and an optical compensation film corresponding to the display is required.
As the conventional optical compensation film, a stretched film of a cellulose-based resin, a polycarbonate, a cyclic polyolefin or the like is used. In particular, a film composed of a cellulose-based resin, such as triacetyl cellulose film, is being widely used because of its good adhesiveness to a polyvinyl alcohol that is a polarizer.
However, the optical compensation film composed of a cellulose-based resin has several problems. For example, although a cellulose-based resin film is processed into an optical compensation film having a retardation value corresponding to various displays by adjusting the stretching conditions, the three-dimensional refractive indices of a film obtained by uniaxially or biaxially stretching a cellulose-based resin are ny≧nx>nz and in order to produce an optical compensation film having other three-dimensional refractive indices, for example, three-dimensional indices of ny>nz>nx or ny=nz>nx, a special stretching method of, for example, adhering a heat-shrinkable film to one surface or both surfaces of the film and heating/stretching the laminate to apply a shrinking force to the thickness direction of the polymer film is required, making it difficult to control the refractive index (retardation value) (see, for example, Patent Documents 1 to 3). Here, nx indicates the refractive index in the fast axis direction (the direction having a minimum refractive index) in the film plane, ny indicates the refractive index in the slow axis direction (the direction having a maximum refractive index) in the film plane, and nz indicates the refractive index in the out-of-film plane direction (thickness direction).
Also, although a cellulose-based resin film is generally produced by a solvent casting method, the cellulose-based resin film deposited by a casting method has an out-of-plane retardation (Rth) of about 40 nm in the film thickness direction and therefore, there is a problem that a color shift may occur, for example, in an IPS-mode liquid crystal display. Here, the out-of-plane retardation (Rth) is a retardation value represented by the following formula:Rth=[(nx+ny)/2−nz]×d (wherein nx represents the refractive index in the fast axis direction in the film plane, ny represents the refractive index in the slow axis direction in the film plane, nz represents the refractive index in the out-of-film plane direction, and d represents the film thickness).
Furthermore, a retardation film composed of a fumaric acid ester-based resin has been proposed (see, for example, Patent Document 4).
However, the three-dimensional indices of a stretched film composed of a fumaric acid ester-based resin are nz>ny>nx, and lamination to another optical compensation film or the like is necessary for obtaining an optical compensation film exhibiting the above-described three-dimensional refractive indices.